1. Field of the Invention
This invention relates generally to the provision of a controlled voltage for the printing element of a printing terminal and more specifically to the use of switching regulator power supplies for the printing element of a thermal printing terminal.
2. Description of the Prior Art
Various types of printing terminals are known including that disclosed in U.S. Pat. No. 3,638,197. Such printing terminals are adapted to provide a hard copy display of information, typically but not limited to alphanumeric information. Certain printing terminals use thermal printheads such as the semiconductor printheads disclosed in U.S. Pat. Nos. 3,496,333 and 3,988,569. Other types of thermal printheads are known and are fabricated using thick or thin film processes. Still other types of printing mechanisms include wire matrix printing elements and inkjet printing elements. Various of these printing elements require the provision of a controlled voltage to effect the printing operation.
In the case of certain types of printheads it is desirable to provide some type of feedback representative of the effectiveness of the printing element. Thus, in the case of semiconductor thermal printheads, as illustrated in U.S. Pat. No. 3,577,137 it is desirable to provide a feedback voltage representative of the operating temperature of the printhead. This feedback voltage is used to control the voltage level of the power supplied to the thermal printhead. The feedback voltage is typically generated by a semiconductor diode located on the printhead structure itself. The nature of the feedback is such that as the printhead temperature tends to move about its nominal value, the voltage supplied to the printhead is reduced thereby tending to maintain the printhead at the desired nominal temperature. In such prior art systems as illustrated by the above referenced patent, a linear type voltage regulator is used to supply power. Because of the requirement for a variable voltage supply to the printhead this linear regulation technique is inefficient. A substantial portion of the power from the power supply is dissipated in the regulating transistors themselves.